Together, DNA repair and checkpoint responses ensure the integrity of the genome Coordination of cell cycle checkpoints and DNA repair is especially important when unusually high loads of DNA damage are sustained following radiation or genotoxic chemotherapy. The checkpoint kinase, Cds1, is activated by ATM in response to DNA damage. Its role as a checkpoint kinase depends on its ability to phosphorylate cell cycle regulators such p53 and Cdc25. Observations in both budding and fission yeast indicate that Cds1 homologues have roles in promoting survival that are independent of their ability to regulate cell cycle progression. Our studies show that human Cds1 physically interacts with Mus81, a conserved UV DNA damage and replication stress tolerance protein that has homology to the XPF- family of endonucleases. Biochemical analysis shows that MusB1 has associated endonuclease activity against structure-specific oligonucleotide substrates, including synthetic Holliday junctions. Mus81-associated endonuclease resolves Holliday junctions into linear duplexes by cutting across the junction exclusively on strands of like polarity. In addition, we find that the abundance of Mus81 protein increases in cells following treatment with agents that damage DNA. Taken together, these findings suggest a role for Mus81 in recombination repair Biochemical, and cell biological techniques will be used to investigate of the exact mechanism by which Mus81 is regulated following checkpoint challenge, the enzymatic properties of Mus81 will be investigated and used to refine predictions of its in vivo function. Loss of function models will be used to assess the importance of Mus81 in the mammalian DNA damage response. As a point of interaction between checkpoint control and DNA repair process the relationship between Mus81 and Cds1 is likely to provide information that is critical to understanding the DNA damage response process as a whole and, thus, aid in the rational development of novel and improved anti-cancer therapies.